A lead frame forms the base or skeleton of an IC package, providing mechanical support to semiconductor dies during assembly into a finished package. A lead frame typically includes a die paddle for attaching a semiconductor die, and leads providing the means for external electrical connection to the die. The die can be connected to the leads by wires, e.g. through wire bonding or tape automated bonds. Lead frames are typically constructed from flat sheet metal, e.g. by stamping or etching. The sheet metal is typically exposed to chemical etchants that remove areas not covered by photoresist. After the etching process, the etched frames are singulated (separated) into lead frame strips. Each lead frame strip includes a number of unit lead frames each having the die paddle and lead construction described above.
Semiconductor dies attached to the die paddles after completion of the assembly process of a lead frame strip are usually tested after separation of the unit lead frames from the lead frame strip, e.g. by punching. Alternatively, the unit lead frames remain mechanically connected to the lead frame strip by tie bars during die testing. This is commonly referred to as lead frame strip testing. Separation of the individual unit lead frames from the lead frame strip occurs after electrical testing. However, the devices must be electrically isolated from one another prior to lead frame strip testing to ensure proper device testing.
Some types of conventional processing involve sawing about half-way through the periphery of each unit lead frame, including through the metal leads, to sever the leads and so that a thin part of molding compound remains intact in the periphery to hold the units in place during lead frame strip testing. However, such processing increases wearing of the sawing blade which can cause inaccuracy. Also, sawing through the metal leads to provide electrical isolation can cause smearing of the metal material which is particularly the case for copper leads. As such longer cycle times are needed to saw through the leads prior to lead frame strip testing, increasing the cost of the individual packages produced from the lead frame strip.
In other conventional approaches, a metal carrier is provided which has an upper surface including an encapsulating matrix with a plurality of sawing lines. A plurality of bump pads and a plurality of die pads are formed on the upper surface of the metal carrier by plating. Semiconductor dies are attached to the corresponding die pads and a plurality of bonding wires connect the bonding pads on the active surfaces of the chips to the bump pads. A square encapsulant covers the encapsulating matrix including the sawing lines and the chips. After etching away the metal carrier, the bump pads and the die pads are exposed from the bottom surface of the encapsulant. Even though the sawing process is done through only the encapsulant and not metal, the cost of the lead frame strip significantly increases by adding the plurality of bumps and die pads, which are formed on the upper surface of the metal carrier by plating. In addition, to form a plurality of encapsulants per single package, lead frame cost further increase as a result of lower material utilization because of larger lateral package size (i.e. encapsulant overhang).